This invention relates generally to apparatus and methods for perforating a well. The invention relates more particularly, but not by way of limitation, to a perforating gun and method suitable for use in, for example, a sand-containing slotted liner located in a non-vertical section of an oil or gas well.
An oil or gas well typically has a lining made of metallic casing or liners cemented in the earthen well bore. If such a lining traverses a hydrocarbon-bearing formation, the lining is perforated to create holes through the metal and cement so that the hydrocarbons can flow from the formation into the well. Perforating conventionally includes lowering an explosive charge-carrying perforating gun into the well and then detonating the explosive charges to blast openings into the lining and wall of the well bore.
Such conventional perforating is suitable for perforating within vertical sections of a well. Conventional perforating may also be useful in non-vertical sections of directionally drilled wells. In either vertical or nonvertical uses, however, conventional perforating can encounter problems from sand entering the well, for example. Such problems can be most apparent when considering a horizontal section of a well as will be done in the following illustrative explanation.
Because gravity does not pull material through a horizontal section as it does through a vertical section of a well, a horizontal section can more readily fill with sand ("sand" as used herein encompasses particulate earthen material and drilling and other debris which can flow or move into a well). This is particularly a problem in a horizontal section of a well lined with a liner which already is slotted with openings to allow hydrocarbon flow, but which openings also allow sand to flow into the interior of the liner. Although a slotted liner allows hydrocarbon flow, it or some other section of the well might still need to be perforated. If such a horizontal section fills with sand before the well can be perforated, the sand can prevent a perforating gun from being located at a desired location. If such a horizontal section fills with sand upon perforating the well, the detonated perforating gun might not be easily retrieved. This latter problem could also occur if the well bore collapses within the horizontal section upon detonation of the perforating gun.
To overcome these problems of being unable to emplace or retrieve a perforating gun, there is the need for an apparatus and method which will allow a perforating gun to be placed at any desired location within a well, even a sand-obstructed section, and to be retrieved once the gun has been detonated.